1. Field of the Invention
This invention relates generally to seismic exploration, and, more particularly, to reducing and extracting wing-tip vortex energy during seismic exploration.
2. Description of the Related Art
Marine seismic exploration is widely used to locate and/or survey subterranean geological formations for hydrocarbon deposit because many hydrocarbon deposits are found beneath bodies of water. FIG. 1 conceptually illustrates a conventional system 100 for carrying out a marine seismic survey. The conventional system 100 includes a survey vessel 105 coupled to a seismic array 110, which typically include one or more streamers 120. One or more seismic sensors 125, such as hydrophones, may be distributed along the length of the seismic streamer 120. Although not shown in FIG. 1, one or more seismic sources may also be included within the conventional system 100.
In operation, the survey vessel 105 tows the seismic array 110 along a predetermined path. One or more seismic sources (not shown) may be used to drive an acoustic wave, commonly referred to as a “shot,” through the overlying water and into the ground. The acoustic wave may be reflected by subterranean geologic formations and propagate back to the seismic sensors 125. The seismic sensors 125 receive the reflected waves, which are then processed to generate seismic data. Analysis of the seismic data may indicate probable locations of geological formations such as hydrocarbon deposits.
The accuracy of the seismic survey is determined, in part, by how accurately the seismic array 110 is towed along the predetermined path, indicated by the arrow 112. Thus, in addition to guiding the seismic array 110 by steering the survey vessel 105, one or more deflector devices 135 are typically coupled to the seismic array 110. The deflector devices 135 are typically used to maintain a spread of the streamers 120 of the seismic array 110 and to steer the path of the seismic array 110 so that it approximately coincides with the predetermined path 112.
One or more hydrofoils 135 are typically used as the deflector devices 135 attached the seismic array 110. The hydrofoils 135 provide lift as they are towed through the water by the survey vessel 105. For example, a hydrofoil 135 used as a deflector device in a seismic survey has a lift of several tons. The lift provided by the hydrofoils 135 is used to maintain a spread of the streamers 120 of the seismic array 110 and/or to steer the path of the seismic array 110. However, the ability of the hydrofoils 135 to maintain the spread of the streamers 120 and/or to steer the seismic array 110 is limited by the electrical power available to the hydrofoil 135. Towed generators (not shown) may be towed by the streamers 120, but these typically only provide a small amount of electrical power while significantly increasing the drag of the streamers 120.
FIG. 2 conceptually illustrates an exemplary embodiment of a conventional hydrofoil 200. The hydrofoil 200 may generate tip vortices 210 near one or more tips 205 of the hydrofoil 200 as the hydrofoil 200 is towed through the water in a direction indicated by the arrow 212. Persons of ordinary skill in the art should appreciate that the path of the fluid flow in the tip vortices 210 typically has a spiral shape, i.e. the fluid flows along a three-dimensional approximately cork-screw-like path, although the tip vortices 210 are conceptually illustrated in FIG. 2 by approximately sinusoidal lines. The tip vortices 210 are regions of highly rotational fluid that trail the tips 205 of the hydrofoil 200. The rotational strength of the tip vortices 210 is approximately proportional to the lift generated by the hydrofoil 200. The tip vortices 210 may have many undesirable consequences for the marine seismic survey. For example, the tip vortices 210 may interfere with objects located in the wake of the hydrofoil 200, such as the seismic streamers 120, the seismic sensors 125, and the like, potentially increasing the noise that may be received on the seismic sensors 125. For another example, the tip vortices 210 may cause one or more of the seismic streamers 120 to oscillate undesirably. The tip vortices 210 may also reduce the lift and increase the drag of the hydrofoil 200. For example, if the hydrofoil 200 has a lift of about ten tons and a lift-to-drag ratio of about five, the drag of the hydrofoil 200 would be about two tons, most of which is related to the energy that goes into the tip vortices 210.
A number of techniques for reducing the undesirable interaction between the conventional hydrofoil 200 and the tip vortex have been proposed. Cordier, et al (International Patent Publication Number WO 00/18641) has described a technique of placing serrated plates on a free edge of a hydrofoil for breaking a tip vortex generated near the free edges of the hydrofoil. Various devices for rectifying flow of a turbulent fluid have also been proposed. For example, a flow rectifier that may be mounted in a duct along which a fluid is flowing has been described by Ligneul (U.S. Pat. No. 5,309,946).
The conventional hydrofoil 200 may also include a rotational body 220, also called a torpedo body, positioned near one or more tips 205 of the hydrofoil 200. For example, the rotational body 220 may be included to increase the buoyancy of the hydrofoil 200. However, the rotational body 220 may also be an effective concentrator of a tip vortex 225, which may result in the tip vortex 225 having a higher rotational velocity than the tip vortex 210. Consequently, the presence of the rotational body 220 may lead to higher downstream disturbances caused by the tip vortex 225.
The present invention is intended to address one or more of the problems discussed above.